Stress-dependent activation entropy in thermally activated cross-slip of dislocations

Significance

A quantitative understanding of the strength and plasticity of crystalline solids requires the ability to predict the rate of thermally activated processes such as dislocation cross-slip. However, current transition-state theory predictions for the rate of dislocation cross-slip are orders of magnitude lower than what is observed in molecular dynamics simulations. This long-standing discrepancy has been challenging to resolve. Here, we show that the discrepancy is caused by the anharmonic effects of thermal expansion and thermal softening, which have been previously neglected. Our findings demonstrate the importance of including these anharmonic effects in the rate predictions of all stress-driven, thermally activated processes in solids.

Abstract

Cross-slip of screw dislocations in crystalline solids is a stress-driven thermally activated process essential to many phenomena during plastic deformation, including dislocation pattern formation, strain hardening, and dynamic recovery. Molecular dynamics (MD) simulation has played an important role in determining the microscopic mechanisms of cross-slip. However, due to its limited timescale, MD can only predict cross-slip rates in high-stress or high-temperature conditions. The transition state theory can predict the cross-slip rate over a broad range of stress and temperature conditions, but its predictions have been found to be several orders of magnitude too low in comparison to MD results. This discrepancy can be expressed as an anomalously large activation entropy whose physical origin remains unclear. Here, we resolve this discrepancy by showing that the large activation entropy results from anharmonic effects, including thermal softening, thermal expansion, and soft vibrational modes of the dislocation. We expect these anharmonic effects to be significant in a wide range of stress-driven thermally activated processes in solids.